An Automatically Switched Optical Network (ASON) is a type of dynamically and automatically switched transport network. It is a new generation of optical networks, wherein a service request is initiated dynamically by users; a path is calculated and selected automatically by a network element; and the setup, restoration, clearance of a connection are controlled by signaling; moreover switching and transporting are integrated.
An ASON includes two layers: a control plane and a transport plane. The function of the control plane includes collecting and distributing the network topology of the ASON to form a “network map” representing the accurate network topology, calculating a viable path with some routing algorithms by use of the “network map”, and establishing an intelligent circuit by driving each node on the path through signaling protocols. The function of the transport plane is to set up or delete cross-connections on each network element, and establish or withdraw services on the transport plane according to instructions from the control plane.
Conventionally, the ASON is mainly applied to a backbone network, in which the services are carried by higher order Virtual Containers (VC), i.e., VC4, and the majority of optical communication equipment manufacturers currently only support intelligent services with VC4 granularity. Along with the popularization and development of ASON, it gradually expands to the Metropolitan Area Network where applications of services with VC12 granularity are growing, therefore, establishing a Label Switched Path (LSP) with VC12 granularity, i.e., a VC12 LSP, becomes an important demand in ASON. RFC 3946 (GMPLS Extensions for SONET/SDH Control) defines a label format for the VC12 LSP. However, as the bandwidth of one VC4 is enough for 63 VC12 LSPs, the number of VC12 LSPs grows so huge that varieties of problems concerning performance emerge, which are unacceptable by equipment manufacturers and network operators. For example, provided there are 48 services with VC4 bandwidth over an optical fiber and each VC4 contains 63 VC12 LSPs, when the optical fiber is interrupted, there will be 48*63 LSPs initiating re-routing processes, which takes long time to recover and thus leads to poor recovery performance; furthermore, in a node with 40 Gbps lower order cross-connection capability, 4*64*63/2=8064 VC12 LSPs may be established, which will occupy a large storage space of the node and lead to the shortage of the storage space.
The above problems may be solved by establishing an end-to-end tunnel or a segment tunnel. However, in the case that an end-to-end tunnel is established, a dedicated VC4 tunnel will be occupied even there is only one VC12 service or just a few VC12 services transmitted through the tunnel, which results in a very low bandwidth utilization rate. In the case that a segment tunnel is established, i.e., VC4 tunnels are established in advance between every two nodes and VC12 services are loaded on these tunnel segments, as every node is the source node and the destination node of an LSP tunnel, when there is a failure in one of the nodes or the optical fibers on both sides of a node are interrupted at the same time, the tunnel LSP cannot perform a re-routing process, hence the VC12 services cannot be effectively protected. In view of the above problems, a perfect VC12 service solution is not put forward by the industry yet.